1. Field of the Invention
This invention relates to circuitry for controlling the value of currents and, more particularly, to methods and apparatus for providing improved current mirroring circuitry.
2. History of the Prior Art
In electronic circuitry, it is often desirable to provide a current which has a value identical to that of current through some other circuit device not in series with the first device. This is typically accomplished through the use of a current mirroring circuit. By maintaining the voltage levels equal at the gate and source terminals of a pair of field effect transistor devices operating in saturation, the current through the devices may be kept equal. There are situations in which the voltage values at various nodes of the current mirroring circuitry must be changed before proper operation of the circuitry can occur. For example, when power is first applied to a circuit including current mirrors, voltages at some circuit nodes exhibiting capacitive loading must be brought to certain values in order for the circuit to function correctly. Often these circuits use only a small amount of current for operational purposes and the current mirroring properties of the circuits are designed to provide these precise small amounts. If only a small amount of current is available, an inordinate amount of time would typically be required for any substantial inherently capacitive load at the node to be charged to change the voltage level.
It is desirable to be able to provide a current mirror circuit which is capable of rapidly changing the voltages at nodes of the circuit exhibiting capacitive loading while mirroring amounts of current smaller than are capable of charging the inherent capacitances at the varying nodes of the current mirror circuit within the time period allowed for the change.